Sound output plays through mic

I've been having an ongoing issue trying to use chat programs such as Skype or Ventrilo on my MacBook Pro. The problem is that no one can make out what I'm saying when I try to talk to them.

I've been using a set of headphones with a built-in mic and volume controls, essentially, the sort of headphones that come with the iPhone. I've used specifically those headphones and other similar headphones designed to work with the iPhone. In the past, this seems to have worked just fine, but now, with my brand-new headphones, my previous headphones and iPhone headphones, I'm encountering the same problem.

It seems like the output sounds, such as the sound coming from a game I'm playing or anything else making sound on my computer, are fed back into the input of my machine. It's as if I didn't have headphones plugged in at all, and the mic was placed next to my speakers. Or if I had intentionally set something on my system so that it would play my computer's sounds into the chat program (something you might want to do, I suppose, if you wanted to play a song or something to the people you were chatting with).

The thing is, there's nothing I can find anywhere to correct this. It didn't use to be a problem, as far as I know, until I launched Skype quite some time ago to chat with a couple other people (I had been successfully using Ventrilo until this). They reported hearing my game sounds, so I dropped the call. I'm not entirely certain I've been completely unsuccessful since then (they may have been able to make out the sound of my voice if I didn't have anything else playing on my computer), but it seems as if the vast majority of the time, at least, I can't be understood on Vent.

Does anyone know of a system setting that could affect this? Does anyone know of a Skype setting that could have messed with my system audio in some way that is now also affecting other applications (Vent)?

I'm comfortable messing around with the Console if there are some commands I can throw at my machine to reset the sound card or some such nonsense, as long as the commands are very explicitly spelled out for me! Also, I have been unable to test this with a USB headset and mic, as my headset is currently misplaced from a move. I should also note that the volume controls on the headphones work fine to control the MacBook Pro's volume, further confirming that it should be compatible with the machine.